The operating conditions in major components in catalytic cracking units are highly interdependent. Coke production is a key factor in the interdependence of the catalytic cracking conditions. To illustrate this point reference is made to G. D. Hobson et al, MODERN PETROLEUM TECHNOLOGY, Applied Science, p 305-308 (1973), which indicates that after considerable experience, it was realized by the petroleum industry that the system was self compensating: "the system was self compensating with regard to heat balance over a wide range of process-operating variables such as feed rate, recycle rate, feed temperature and reactor temperature. This occurs because of the effect of catalyst/oil ratio on conversion, the effect of conversion on the coke yield and the method of reactor temperature control. A rise in feed temperature, for example, will change the heat balance of the reactor so that the reactor temperature tends to rise. As a result, the temperature controller will reduce the catalyst flow from the regenerator, which reduces the catalyst/oil ratio. The effect of this is to reduce the conversion which in turn reduces the yield of coke and thus the heat release in the regenerator. The unit readjusts itself to remain in heat balance at a slightly lower conversion, which can then be restored to its previous value by a slight rise in reactor temperature." The author, G. D. Hobson, explained by way of example: When an increase in the feed temperature occurs a rise of reactor temperature to maintain conversion is required and with a drop of catalyst circulation; the regenerator/reactor temperature differential has increased and the coke yield dropped, which is an important factor in units where coke-burning capacity is a limiting factor. More feed can then be processed to give the same weight of coke production per hour. The reduction in coke yield is due partly to the fact that at a given conversion the coke yield tends to fall as the temperature increases, and partly to the lower catalyst circulation rate which reduces the quantity of entrained hydrocarbon vapours entering the regenerator with the catalyst and being burnt.
In fluidized catalytic cracking processes, a relatively heavy hydrocarbon feedstock, e.g., a gas oil, admixed with a suitable cracking catalyst to provide a fluidized suspension, is cracked in an elongated reactor, or riser, at elevated temperature to provide a mixture of lighter hydrocarbon products. The Basiform reaction products and spent catalyst are discharged from the riser into a separator, e.g., a cyclone unit, located within the upper section of an enclosed stripping vessel, or stripper, with the reaction products being conveyed to a product recovery zone and the spent catalyst entering a dense catalyst bed within the lower section of the stripper. In order to remove entrained hydrocarbon product from the spent catalyst prior to conveying the latter to a catalyst regenerator unit, an inert stripping gas, e.g., steam, is passed through the catalyst where it desorbs such hydrocarbons conveying them to the product recovery zone. The fluidizable catalyst is continuously circulated between the riser and the regenerator and serves to transfer heat from the latter to the former thereby supplying the thermal needs of the cracking reaction which is endothermic.
Some catalytic cracking systems in current operation employ large pore crystalline silicate zeolite cracking catalysts in preference to the earlier used amorphous silica-alumina cracking catalysts. These relatively new crystalline zeolite cracking catalysts, e.g., zeolites X and Y, are generally regarded as low coke producing catalysts. Thus, as the level of coke deposits has been reduced through the use of crystalline zeolite cracking catalysts, it has been equally important to concentrate on recovering the maximum amount of heat available through the burning of deposited coke in the regenerator.